Generally, a polyimide (PI) film is formed of a polyimide resin. Here, a polyimide resin is a high heat-resistance resin prepared by a process including the steps of: solution-polymerizing an aromatic dianhydride with an aromatic amine or an aromatic diisocyante to obtain a polyamic acid derivative; and ring-closing and dewatering the polyamic acid derivative at high temperature to imidize the polyamic acid derivative. In order to prepare a polyimide resin, pyromellitic dianhydride (PMDA), biphenyltetracarboxylic dianhydride (BPDA) or the like is used as an aromatic dianhydride, and oxydianiline (ODA), p-phenylenediamine (p-PDA), m-phenylenediamine (m-PDA), methylenedianiline (MDA), bisaminophenylhexafluoropropane (HFDA) or the like is used as an aromatic diamine.
Such a polyimide resin, which is an unmelted ultrahigh heat-resistance resin, has been widely used in the field of high-tech heat-resistant materials, such as automobile materials, aircraft materials, spacecraft materials and the like, and electronic materials, such as insulation coating agents, insulation films, semiconductors, electrode protection films for TFT-LCDs and the like, because it has excellent oxidation resistance, heat resistance, radiation resistance, low-temperature characteristics, chemical resistance and the like. Recently, a polyimide resin has also been used in the field of display materials, such as optical fibers, liquid crystal oriented films and the like, and transparent electrode films by adding conductive filler to the polyimide resin or coating the surface of the polyimide film with conductive filler.
However, a polyimide resin is problematic in that its transparency in a visible light region is low because it is colored brown or yellow due to its high aromatic ring density, in that its light transmittance is low because it provides a yellow color, and in that it is difficult to use it as an optical material because it has high birefringence.
In order to solve such a problem, a method of refining monomers and a solvent and then polymerizing the refined monomers has been attempted, but this method is also problematic in that light transmittance is not greatly improved.
U.S. Pat. No. 5,053,480 discloses a method of preparing a polyimide resin using an aliphatic cyclic dianhydride instead of an aromatic dianhydride. This method is advantageous in that, when the polyimide resin prepared by this method is formed into a solution phase or a film, its transparency and color is improved, but is problematic in that the improvement of transparency thereof is restricted and thus satisfactory transparency cannot be obtained, and in that thermal and mechanical characteristics thereof are deteriorated.
Further, U.S. Pat. Nos. 4,595,548, 4,603,061, 4,645,824, 4,895,972, 5,218,083, 5,093,453, 5,218,077, 5,367,046, 5,338,826, 5,986,036 and 6,232,428 and Korean Unexamined Patent Application Publication No. 2003-0009437 reported a novel polyimide structure, the light transmittance and color transparency of which is improved by using a monomer having a bent structure whose m-site, rather than its p-site, is connected with a functional group such as —O—, —SO2—, CH2— or the like, an aromatic dianhydride having a substituent group such as —CF3 or the like and an aromatic diamine monomer on the condition that its thermal characteristics are not greatly deteriorated. However, they reported that the tear strength of the novel polyimide structure is 7 N/mm or less, which is insufficient.